Positron emission tomography (PET) is a modality of nuclear medicine for imaging metabolic processes using gamma photons emanated from radiopharmaceuticals ingested by a patient or injected into a patient. Multiple PET images are taken in multiple directions to generate/reconstruct a 3-dimensional PET image and/or multiple slices of a PET image. Before image reconstruction, raw PET data are in projection/sinogram space. PET scanning generally provides useful information regarding the functional condition of the body tissues and systems such as the cardiovascular system, respiratory system, and/or other systems. PET scanning is useful for indicating the presence of soft tissue tumors or decreased blood flow to certain organs or areas of the body.
PET scanners with cylindrical geometry can have a reduction in radial spatial resolution, which increases with increasing distance from the center of the field of view (FOV) of the scanner. The loss is due to a parallax effect, which is in turn due to uncertainty in determining a position of a positron-electron annihilation event (“gamma event”) with respect to the line of response (LOR) that joins the scintillators involved in the interaction. When a source is relatively far from the central axis, the difference in position between the true line of flight (LOF) of the photon pair and the estimated LOR can be large. Thus, improvement in depth-of-interaction (DOI) measurement is desirable.
There are several methods to attain DOI information. Many approaches are based on a “stacked geometry” of the PET detector block. Two or more layers of scintillator arrays are provided. The gamma rays from a positron-electron annihilation event are absorbed by one of the scintillators, and cause emission of photons. DOI information can be obtained by determining in which layer of scintillator a gamma ray was actually absorbed.